The issue here might be 2-fold. Traveling on land to ford, during ice, allows you to zoom out on the land portion of the water tile.
Last zoom-out location is stored in the zoommaps. Re-entering that tile on water, will place the character on the last 'zoom out' spot. Ice weakening, the game doesn't let you exit the tile on land portion of the water tile. (same as with lake&sea shore, river banks)
Zooming out, on water while on watercraft, or during reliable ice should save the zoom out spot on to the zoommaps.
Of course, it could be error/corruption on the zoommaps, not updating the exit spot. (it won't save the exit if you walk over to adjacent tile and zoom-out there)